Coupling from a resonant cavity through a dielectric window to an external loop



April 28, 1964 D. R. ARMSTRONG ETAL 3,131,326 COUPLING FROM A RESONANT CAVITY THROUGH A DIELECTRIC WINDOW TO AN EXTERNAL LOOP Filed Aug. 5, 1960 I8 IIIII I Hill"! I J '1 I ll 2 44 CD. i Q

*INVENTORS DESMOND R. ARMSTRONG CARL H. ITURNBLOM eh) M Mix 8% ATTORNEYS United States Patent 3,131,326 COUPLING FROM A RESONANT CAVITY THROUGH A DIELECTRIC WINDOW TO AN EXTERNAL LOOP 3 Desmond R. Armstrong, San Carlos, Calif., and Carl H. Turnlilom, Salt Lake City, Utah, assignors to Eitel- McCullough, Inc., San Carlos, Calif., a corporation or California Filed Aug. 5, 1960, Ser. No. 47,865 4 Claims. (Cl. 315-39) This invention relates to the art of coupling high-frequency electromagnetic energy from a resonant cavity.

High-frequency electromagnetic energy is coupled from a resonant cavity either by an iris which opens into a waveguide, or by a coupling loop which is connected to a coaxial line.

In relatively low-frequency electron tubes incorporating resonant cavities, the cavities are usually made with a cylindrical dielectric vacuum wall disposed coaxial. with the tube axis, thereby placing a portion of each cavity external of the vacuum wall as taught in Patent No. 2,619,611 to Norton. In these tubes, energy can readily be coupled from the external portion of the cavity. The same tube can be used in connection with either a waveguide circuit or a coaxial circuit since the coupling means is in the external portion of the cavity and the external portion is readily detachable from the tube.

In relatively high-frequency electron tubes incorporating' cavities which resonate at frequencies of over 10,000 megacycles, a cylindrical dielectric vacuum wall coaxial with the drift tube is not practical because high-frequency cavities dictate that they be of small volume. Energy is coupled from a high-frequency cavity through a small loop which connects to a coaxial line and protrudes through a small aperture in the cavity. A suitable vacuum seal placed within the coaxial line maintains the vacuum. in the tube and cavity. If one desired to insulate the loop and coaxial line from the cavity, a small thimble-shaped dielectric member would be inserted into the aperture with the rim of the thimble sealed to the cavity. Then the loop would be inserted into the well of the thimble and would be disposed within the cavity. The power would be coupled therefrom. This method has drawbacks in that electromagnetic energy is lost through radiation and therefore the efficiency of the system is impaired. Also, since the thimble protrudes into the cavity, the volume of the cavity must be larger than that of a cavity with no thimble. Therefore, the cavity with the thimble disposed therein will resonate at alower frequency than will a smaller cavity with no thimble.

Higher frequency cavities have been made with their apertures sealed over by a flat dielectric window. This arrangement was primarily used to couple a waveguide to the cavity so that the high-frequency electromagnetic energy within the cavity would radiate through the window and into the waveguide. One end of the waveguide was permanently brazed to the tube by its end plate encircling the window over the aperture. Then, if one wanted to insulate the waveguide from the cavity, a thin insulation gasket was placed between any two connecting flanges of the waveguide.

Up to now a. high-frequency tube was only adapted for either a waveguide circuit or a coaxial circuit. A basic alteration had to be made to the tube if one wanted to use the tube so that it coupled into a waveguide instead of a coaxial line. Consequently, manufacturers and suppliers were required to stock two tube types.

An object of this invention is to provide a high-frequency resonant cavity which can be coupled to either a waveguide circuit or a coaxial circuit.

Another object of this invention is to provide improved D.C. insulation between a high-frequency resonant cavity and an accompanying circuit.

Yet another object of this invention is to provide a means which will couple electromagnetic energy through a flat window in the wall of a resonant cavity directly into a coaxial line.

Briefly described, the novel means for coupling highfrequency electromagnetic energy into or out of a cavity comprises a flat dielectric window sealed within an aperture (iris) formed in the wall of the cavity. A novel shaped loop which is mounted on the end of a coaxial line is placed adjacent the window, whereby the loop couples the coaxial line and cavity together. A flange may be disposed on the end of the outer conductor of the coaxial line for mounting the loop against the window, and then by the use of a thin dielectric gasket between the flange and cavity there is formed an eflicient D.C. insulated coaxial line whichis coupled to the cavity. A waveguide is coupled to the same cavity and through the same aperture by removing the coaxial line and mounting an adapter plate around the window to which plate a waveguide would be mounted.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of the invention. The invention is not limited to the disclosed embodiment but includes variant embodiments thereof within the scope of the claims.

Referring to the drawing:

FIGURE 1 is an outline of a high-frequency electron tube illustrating the dielectric window (partially removed) covering the aperture.

FIGURE 2 is a cross-section taken on line 2-2 of FIG- URE 1 with the novel loop coupler of a coaxial. line mounted thereto.

FIGURE 3 shows a broken-out section of the end view of the tube coupled to a waveguide shown in cross-section.

Referring to FIGURE 1 in particular, there is shown the reflex klystron which can be adapted either to coaxial coupling means or to a waveguide coupling means through the aperture 10 which has a dielectric window 11 sealed vacuum-tight within a window cup 12. v

FIGURE 2 shows a cross-section taken on line 22 of the reflex klystron of FIGURE 1 in which is incorporated a novel coaxial coupling means 13. The reflex klystron has an electron gun 14 disposed at one end and a repeller 16- at the other end, and between the two is disposed a resonant cavity 17 with an interaction gap 18. Electro magnetic oscillations are formed in the resonant cavity 17 in the following manner: The gun 14 projects a beam. of electrons through the interaction gap 18 of. the cavity 17, whereby, by well-known electronic principles, the electrons in the beam become velocity'modulated. The electrons are then repelled by the repeller 16 and are caused to reenter the gap 18 in bunches. The electron bunches transfer their energy into electromagnetic energy causing stronger electromagnetic oscillations within the cavity. A reflex klystron is only one of the means known for producing electromagnetic energy in a resonant cavity. This embodiment of the invention is shown in conjunction with a reflex klystron, but other apparatuses with resonant cavities can be combined with the novel features of this invention. A mounting flange 19 is used for mounting the tube in place.

Electromagnetic energy radiates from the cavity 17 through the window 11 and the coaxial coupling means 13 transfers the energy into a coaxial line in the following manner: The window 11 is flat and is sealed by a sealing flange 21 within the window cup 12 and over the aperture 10. The inner conductor 22 of the coaxial line is fixed to one end of a loop 23, which has the other end brazed to the outer conductor 24. The ends of the outer and inner conductors are shown in this embodiment so that a standard coaxial plug can be threaded thereto. An insulating bushing 26 supports the inner conductor 22 within the outer conductor 24. The outer conductor has a flange 27 which is fastened by suitable bolts 28 threaded into holes 29 on surface 31 (FIGURE 1) through which aperture is formed. An insulating gasket 32 D.C. isolates the coaxial line from the klystron. In order to have the loop 23 efficiently coupled to the cavity 17, the loop 23 has a straight portion 33 which is placed close and parallelto the window 11 and is oriented substantially parallel to the electron beam path so that the oscillating magnetic field, which is concentric to the beam axis, interacts with the loop.

Referring to FIGURE 3, the broken-out section, end view, shows the window 11 brazed within the window cup 12 and substantially flush with surface 31. FIGURE 3 shows the arrangement in which the coaxial line fitting of FIGURE 2 is removed, and instead a waveguide fitting is attached to the flat surface 31 of the basic tube shown in FIGURE 1. By unbolting the flange 27 of the coaxial line from surface 31, a waveguide 34 is mounted to surface 31 by first attaching a waveguide adapter 36 with suitable bolts 37 threaded into holes 29 on surface 31 and then attaching the waveguide 34 with suitable bolts 38 and nuts 39 to the adapter 36. The adapter 36 has a central opening 41 through which the electromagnetic energy passes from the window 11 into the waveguide and has a matching plug 42 which, in this embodiment, is a metal rod oriented perpendicular to the axis of the waveguide and protruding through the side wall of the adapter 36. Of course, the matching plug 42 is adjustable so that it may move in and out of the adapter through the side wall and match the circuit to the cavity as required for proper operation. An insulating gasket 43, as mentioned above, can also be used in this invention to DC. isolate the waveguide from the cavity potential. Of course, suitable insulating sleeves 44 are placed around the bolts 38.

As described above, when energy is coupled from the cavity 17 into a waveguide, the window 11 should have its exterior surface substantially coplanar with surface 31. The window should not protrude from the surface since it will interfere with the structure of the waveguide and it will be vulnerable to damage when the waveguide is removed. The window should not be recessed into the surface whereby a chamber is formed between the aperture 10 in the klystron and the central opening 41 in the waveguide adapter, which chamber may resonate. In the embodiment described, although a chamber is formed between aperture 10 and opening 41, by making the ext erior surface of the window substantially flush with surface 31 this chamber will not resonate and it becomes an integral part of the coupling mechanism. Of course, the ideal arrangement is to have aperture 10 and opening 41 coplanar, but this arrangement will leave the window 11 exposed when the waveguide is removed and, as mentioned above, the window will be vulnerable to damage. Besides, the waveguide structure would be more intricate to accommodate the window.

When energy is coupled from the cavity 17 into a co axial line, again the preferred arrangement is that the window 11 should have its exterior surface substantially coplanar with surface 31. Again, as before, if the window protrudes it will be vulnerable to damage when the coaxial line is removed. If the window is substantially recessed into the klystron, a chamber 44 which encloses the loop 23 will be of such a size that it will also form frequencies which are undesirable because these frequencies are close to the operating frequency of the klystron. The undesirable frequencies will then be difficult or impossible to suppress, because a loop is a broad band coupling device. Therefore, when a coaxial line is coupled to the klystron, the reasons why the exterior surface of the window 11 should be substantially coplanar with surface 31 become more imperative than when a waveguide is coupled to the klystron. Thusj by making the window with its exterior surface substantially coplanar with the surface 31, chamber 44 ismade as sinallas practical whereby any possible undesirable frequencies formed will not be allowed to resonate. The undesirable frequencies will be suppressed and not be coupled by the loopf Also, since chamber 44 must be of sufiicientsize to house the loop 23 and sincethe cavity 17 must be very small because this is a high-frequency device, the chamber may be large enough to resonate near the operating frequency of the klystron. By placing the loop as close as possible to the aperture 16 without an intervening compartment, the chamber 44 in enclosing the loop becomes a part of the coupling mechanism and not a resonant cavity.

There is also a possibility, if chamber 44 is made too large if the window 11 is recessed into the surface, whereby the chamber 44 will resonate at the operating frequency of the klystron by forming one or more modes within the chamber. If this is the case, then the most efficient place to couple energy from the chamber 44 through a loop will not be adjacent the window 11. The loop must then be moved so that it is in the region where the magnetic field is the strongest, as determined by the location of the modes. I i

The drawings show the exterior of the surface of Window 11 slightly recessed into the surface 31, although the most desirable structure is to have the surfaces coplanar. In an actual tube operating at a frequency of 4300 megacycles, the amount the surfaces were nonplanar was onesixteenth of an inch. The window was recessed this amount to prevent accidental breakage to the window, since the tube will be usedunder rugged environmenta conditions.

In this application the clause the exterior surface of window 11 is substantially coplanar with said fiat exterior surface 31 when used in the claims is limited to mean that the chamber formed outside the aperture 10, when either the coaxial line or the waveguide is coupled thereto, is of such a size that any unwanted frequencies formed within the chamber are suppressed and that one or more modes are not formed within the chamber at the operating frequency of the klystron.

In this embodiment the chamber 44 is filled with a light density plastic foam so that under severe shock conditions the loop will not vibrate and affect the coupling characteristics of the tube.

We claim:

1. A reflex klystron comprising envelope walls forming a tubular body with an electron gun disposed at one end, a repeller disposed at the other end, a resonant cavity having an electron interaction gap disposed between said gun and repeller, one of saidwalls being disposed around said resonant cavity and having a recess in its outer surface, an aperture in the bottom of said recess communicating with said cavity, a flat dielectric window in said recess and hermetically closing said aperture, coupling means located outside of said cavity and adjacent said window for coupling electromagnetic energy out of said cavity, said means including at least a cup-shaped metal line member mounted over said window with the open end of the cup facing the window, a center conductor insulatingly sealed in the closed end of said cup-shaped member, and a conducting loop connectedto said center conductor and the side of said cup and having a flat portion adjacent the outer face of said window.

2. A reflex klystron as claimed in claim 1 in which said one wall has a flat exterior surface around said recess, and the outer face of said window does not extend beyond said flat exterior wall surface.

3. A reflex klystron as claimed in claim 2, in which said cup contains a dielectric plastic filling.

4. An electrical coupling comprising a cup-shaped member having an outwardly projecting attachment rim, a center conductor insulatingly sealed in the closed end of said cup-shaped member, a conducting loop connected to said center conductor and the side of said cup and having a fiat portion positioned outside the open end of said cup-shaped member, and a dielectric plastic filling within said cup.

References Cited in the file of this patent UNITED STATES PATENTS Sensiper May 23, 1950 Hansen et a1. Dec. 18, 1951 Field Nov. 29, 1955 Dodds Aug. 7, 1956 Gardner et a1. Dec. 20, 1960 

4. AN ELECTRICAL COUPLING COMPRISING A CUP-SHAPED MEMBER HAVING AN OUTWARDLY PROJECTING ATTACHMENT RIM, A CENTER CONDUCTOR INSULATINGLY SEALED IN THE CLOSED END OF SAID CUP-SHAPED MEMBER, A CONDUCTING LOOP CONNECTED TO SAID CENTER CONDUCTOR AND THE SIDE OF SAID CUP AND HAVING A FLAT PORTION POSITIONED OUTSIDE THE OPEN END OF 